1. Field of the Invention
The present invention relates to a technique of suppressing degradation of image quality caused by a foreign substance sticking to the surface of, for example, an optical low-pass filter in an image capturing apparatus using an image sensor such as a CCD or CMOS sensor.
2. Description of the Related Art
In recent years, many image capturing apparatus that generate an image signal by using an image sensor such as a CCD and record it as data are on the market, including digital cameras and digital video cameras. A digital camera requires no sensitive film that is conventionally used as a recording medium and instead records image data on a data recording medium such as a semiconductor memory card or hard disk device. Such a data recording medium allows repeated write and erase, unlike a film, and is very convenient due to a large reduction in the cost of related expendables.
A digital camera usually has an LCD (Liquid Crystal Display) monitor device capable of displaying a captured image, as needed, and a detachable mass storage device.
Use of a digital camera having these two devices makes it possible to capture an image without using film, which conventionally serves as an expendable recording medium, to display the captured image on the LCD monitor device and to confirm its content immediately. It is possible to erase undesired image data on the spot or capture another image, as needed. When compared to a silver halide camera, the recording efficiency is much higher.
The scope in which digital cameras are being used is widening due to their convenience as well as to technical innovations, for example, which increase the number of pixels of an image sensor. There have also been recent increases in the number of digital cameras with interchangeable lenses, such as those of single-lens reflex cameras.
However, in digital cameras, foreign substances such as dust or dirt (to be simply referred to as dust hereinafter) sometimes stick to the surface of, for example, an image sensor, an image sensor protective glass fixed on the image sensor, or to an optical filter or an optical system (to be referred to hereinafter as an image sensor/optical system component). If dust sticks to an image sensor/optical system component, the dust shields light and impedes image capture of that part, degrading the quality of the captured image.
Not only digital cameras but cameras using silver halide film also have the problem of dust being captured on film. However, since the film moves with every frame, the same dust is rarely captured on all frames.
However, the image sensor of a digital camera does not move, and image capture is executed using the same image sensor. Once dust sticks to an image sensor/optical system component, the same dust is captured on many frames (captured images). In particular, a lens-interchangeable digital camera readily catches dust in it upon lens exchange.
Hence, the photographer must always take care not to cause dust to stick to the image sensor/optical system component and must expend great effort to check for dust and remove it. In particular, the image sensor is located at a relatively deep point within the camera, and it is not easy to check and remove dust on it.
Dust easily enters a lens-interchangeable digital camera when lenses are exchanged. Many lens-interchangeable digital cameras have a focal plane shutter in front of the image sensor and it is easy for dust to stick to the image sensor/optical system component.
Dust on the image sensor normally sticks not to the surface of the image sensor but to the surface of the protective glass or optical filter. Hence, the imaging state changes depending on the aperture value of the photographing lens or the distance of the pupil position. More specifically, when the aperture value is close to a full-aperture state, defocusing occurs to make small dust unnoticeable. Conversely, when the aperture value is large, focusing on dust occurs and has an adverse effect on the image.
There is known a method of making dust unnoticeable, in which, for example, a white wall is captured in a stopped-down-aperture state to prepare an image containing only dust on the image sensor in advance, and the image is used in combination with a normal image (Japanese Patent Laid-Open No. 2004-222231). With this method, however, the user must always be aware of the correspondence between the image captured for dust detection and the group of actually captured images to be associated with the image; the result is cumbersome.